1. Field of the Invention
The invention concerns a torque sensing unit and a power tool provided with such torque sensing unit by which an output torque responsive signal is generated by a means sensing the reaction torque transferred from a part of a reduction gearing to the tool housing.
In particular, the invention relates a torque sensing unit comprising a reaction torque transferring element connected between a ring gear of a planetary reduction gearing and the tool housing and provided with strain sensors arranged to deliver signals in response to the transferred reaction torque and, hence, the output torque of the tool.
2. Description of the Related Art
One previously known type of torque sensing means is described in for instance U.S. Pat. No. 4,620,449 and comprises an elastically deformable sleeve connecting a ring gear of the reduction gearing to the tool housing, and a strain gauge type sensor means arranged to indicate elastic torsional deformation of the sleeve responsive to the reaction torque transferred to the tool housing. This device is disadvantageous in that it requires a substantial axial space in the tool housing. It is also relatively costly to produce.
Another type of torque sensing means for a torque delivering power tool is described in U.S. Pat. No. 5,172,774. This known torque sensing means comprises a wheel shaped torque transferring element which comprises a hub formed with internal splines for connection to an immobile disc mounted in the housing, and a rim portion formed with external splines for connection to a ring gear element exposed to a reaction torque. This device is advantageous in that it requires a relatively short axial space in the tool housing, but instead it requires a large radial space and is rather complicated and expensive to manufacture.